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What Causes a Solar Eclipse
A solar eclipse can only occur when the moon is in its new phase, which occurs approximately once a month. During this phase, the moon is between the Earth and the sun. The moon casts two shadows: a fainter, wide outer shadow called the penumbra, and a narrow, dark inner shadow called the umbra. During a solar eclipse, people within the path of the umbra will experience a total eclipse, when the entire sun is covered by the moon's shadow. People in the path of the penumbra will experience a partial eclipse, when the sun is only partially blocked by the moon.
Annular and Hybrid Eclipses
The moon's orbit around the Earth is elliptical, not round. Because of this, at certain points in its orbit the moon's distance from the Earth varies. When it is at the far point of its orbit, the moon appears to be smaller, and if a solar eclipse occurs at this time, the moon's shadow is not large enough to block the sun entirely. Such an eclipse is called an annular eclipse. When it occurs, the edge of the sun will still be visible from Earth. Occasionally, an eclipse may be annular in places and total in other parts of the eclipse path. This is called a hybrid eclipse. It is caused when the curvature of the Earth brings certain parts of the eclipse path into the moon's umbral shadow.
How to Watch a Solar Eclipse
It is not safe to look directly at the sun during a partial or annular eclipse. According to NASA, you can look at the sun only during the complete darkness of a total eclipse, but watching a partial eclipse without eye protection may cause blindness or permanent damage to your sight. Although the light may not be painfully bright, there are still infrared and ultraviolet rays that can damage your eyes. You can watch an eclipse using a pinhole to project an image of the sun onto a screen. You can also watch an eclipse through special vision filters that protect your eyes from the sun's rays, such as No. 14 welder's glasses or aluminized mylar.
Frequency of Solar Eclipses
Because the moon's orbit is tilted about 5 degrees from the Earth's orbit around the sun, solar eclipses do not occur every month at the new moon. They can only occur when the Earth, moon and sun line up at the new moon. Also, a total solar eclipse can only be seen from the path of totality, which is the path the moon's umbral shadow follows along the Earth. It is only about 100 to 200 miles wide. However, a partial or total eclipse occurs somewhere on Earth two to five times per year.